Thousands gathered on Sunday at the grave of prominent Cambodian critic Kem Ley who was gunned down a year ago in a murder that sparked widespread anger and scepticism over the alleged killer's motives.

(AFP)

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Thousands gathered on Sunday at the grave of a prominent Cambodian critic who was gunned down a year ago in a murder that sparked widespread anger and scepticism over the alleged killer's motives.

Kem Ley, a popular and charismatic political analyst, was shot twice in the head as he sipped coffee in Phnom Penh -- a brazen assassination that sent shockwaves through the country's already beleaguered activist community.

Unemployed former soldier Oeuth Ang admitted carrying out the killing and was sentenced to life in March after a brief trial.

His declared motive, that the murder was revenge for an unpaid $3,000 debt, caused broad disbelief and was not cross examined in court because he effectively admitted his guilt.

"I don't know whether they made up that debt story but I don't believe it at all," Kem Ley's 77-year-old mother Phok Se, told AFP as well-wishers began gathering at the family home in Takeo province.

People pose for a picture with a statue of Cambodian critic Kem Ley during the first anniversary of his murder

(AFP)

"There has been no justice for us so far," she added, echoing the sentiments of many at the ceremony which saw Buddhist monks chant prayers as devotees made offerings around the grave.

In Phnom Penh police stopped mourners from placing flowers at the petrol station cafe where Kem Ley was gunned down.

In a joint statement to mark the anniversary more than 100 local and foreign organisations called on the government to reopen the case after a "flawed trial".

"There has been no transparency in the murder investigation and there are still many unanswered questions in this case," the statement read, adding there was "compelling evidence" Oeuth Ang had accomplices.

Many friends and supporters find it hard to believe Oeuth Ang, who rarely held down a job, could afford to lend $3,000 -- more than twice the average annual salary in Cambodia.

"One year has passed and yet we are nowhere near uncovering the full picture of what happened to Kem Ley," Chak Sophea, from the Cambodian Center for Human Rights, told AFP.

Cambodia has been ruled by strongman premier Hun Sen for 32-years and has a dark history of usually unsolved activist killings.

In the 1990s and early 2000s such assassinations were common, but they had become rarer in recent years.

Spooked that the bad days may have returned, tens of thousands turned out for Kem Ley's funeral in scenes that rattled the government.